Maraging steel



United States Patent 3,313,662 MARAGING STEEL Frank A. Malagari, Jr., Freeport, Pa., assignor t0 Allegheny Ludlum Steel Corporation, Brackenridge, Pa, a

corporation of Pennsylvania No Drawing. Filed Aug. 20, 1964, Ser. No. 390,994

5 Claims. (Cl. 148-31) This invention relates to maraging steels, and in particular to maraging steels which are characterized by a high degree of toughness and an excellent impact strength in the aged condition.

Recent technological advances have witnessed the development of a relatively new family of steels which have been referred to commercially as maraging steels. In principal, these steels possess a mar-tensitic microstructure and attain increased strength by an age hardening phenomenon being superimposed on the basic martensitic matrix. These steels have resulted in a wide degree of attainable mechanical properties which have proved to be quite desirable from the design engineers standpoint. It has been found, however, that while outstanding mechanical properties, such as tensile strength, yield strength and ductility have been attainable in these steels by employing a relatively simple heat treatment, nonetheless, these steels appear to be deficient from the standpoint of their impact strength in the aged condition.

Predominantly, the presently known and used maraging steels characteristically employ a chemical composition which includes a high nickel content, and wherein the elements titanium and/or aluminum are employed as the age hardening components. In order to alleviate the deficiencies as relates to the lack of impact strength, the composition of the present invention is characterized by a substantially lower nickel content and .by employing the element molybdenum in an age hardening reaction in order to obtain a desirable level of tensile properties. A critical amount of vanadium is also employed in order to obtain an outstanding combination of impact strength with tensile properties.

An object of the present invention is to provide a maraging steel having a high degree of impact strength.

Another object of the present invention is to provide maraging steel which contains a critical amount of vanadium.

A further object of the present invention is to provide a nickel-cobalt-molybdenum maraging steel which contains a critical amount of vanadium.

A more specific object of the present invention is to provide a maraging steel containing nominally about 9% nickel, about 7% cobalt and about 5% molybdenum, and which contains a critical amount of carbon, manganese, silicon and vanadium.

Other objects of this invention will become apparent to those skilled in the art when read in conjunction with the following description and claims.

In its broader aspects, the steel of the present invention contemplates a composition which includes up to 0.05% carbon, up to about 0.2% manganese, up to 0.1% silicon, from about 8% to about 12% nickel, from about 6% to about 9% cobalt, from about 3.3% to about 6.6% molybdenum, from about 0.10% to about 1.0% vanadium and the balance essentially iron with incidental impurities. Within this broad compositional range, it is desired to maintain the alloying components within their respective ranges for the various reasons which will appear more fully hereinafter.

It is desired to maintain the carbon content as low as possible within practical limited limits, and in no event should the carbon content exceed about 0.05%. While higher carbon contents exhibit a fair degree of ductility, the impact strength is adversely affected where the carbon content exceeds 0.05%. The optimum combination of Patented Apr. 11, 1967 tensile properties and impact strength appears to be obtained when the carbon content is limited to about 0.03% maximum. Manganese and silicon produce a pronounced effect on the impact strength of the steel of the present invention, and accordingly, it is desired to maintain the manganese and silicon contents at no greater than about 0.25% in sum total. While the silicon and manganese contents will be present in normally air melted materials, vacuum melting can be accomplished with only traces of these elements being present in the final composition. Accordingly, where air melting is employed, it is desired to limit the sum of the manganese and silicon to about 0.25% maximum, and preferably it is desirable to maintain individually the manganese content up to about 0.2% maximum and the silicon content up to about 0.1% maximum. As will appear more fully hereafter, increasing the sum of the manganese and silicon to more than about 0.25% seriously adversely affects the impact strength exhibited by the steel of the present invention.

It is preferred to maintain the nickel content at about 9% in the steel of the present invention. While the nickel content may vary between about 8% and about 12% the level of the attainable mechanical properties is not significantly different from that obtained where the nickel content is maintained at about 9%, that is to say that nickel contents in excess of 9% up to about 12% do not significantly improve the tensile or impact properties compared with those obtained where the nickel content is maintained at about 9%. While higher tensile properties with lower impact strength can be achieved in teels containing in excess of about 12% nickel, these properties are obtained through the combined effect of heat treatment and increased nickel content. While steels containing lower nickel contents will age harden, the degree of hardness attainable in these steels is correspondingly decreased with decreasing amounts of nickel under 8%. Moreover, the aging temperature to reach maximum hardness was also found to decrease with an increase in the nickel content so that nickel contents in excess of 12% attained substantially the same hardness after aging at 1000 F., as the 9% nickel steel at a lower aging temperature. Accordingly, it is desired to maintain the nickel content within the range between 8% and 12% and optimum properties appear to be obtained where r the nickel content is maintained at about 9% As set forth hereinbefore, the cobalt content of the steel of the present invention is maintained within the range between about 6% and about 9%. Cobalt contents of less than about 6% decrease the level of the attainable age hardened properties whereas cobalt contents up to about 9% retain excellent impact strength. Further increases in the cobalt content do not appear to significantly increase either the impact strength or the tensile properties exhibited by the steel of the present invention. Optimum results appear to be obtained where the cobalt content is maintained at about 7%.

The steel of the present invention requires the presence of a minimum of 3.3% molybdenum in order to promote both age hardening and good impact strength. While some age hardening has been noted with molybdenum contents of less than about 3.3%, the impact strength f these steels is adversely affected. Increasing the molybdenum content up to about 6.6% is effective for producing higher age hardness values and higher tensile properties. However, increasing the molybdenum content to more than about 6.6% again results in a decrease in the impact strength exhibited by the steel of the present invention. Accordingly, it is desired to maintain the molybdenum content at about 5% for optimum results.

The effect of vanadium on the impact strength of the 9% nickel-7% cobalt-5% molybdenum is outstanding. In this respect, a marked increase in the impact strength isnoted where the steel of the present invention contains at least 0.10% vanadium. Presence of a minimal amount of v-anadium, that is about 0.10%, is effective for increasing the impact strength 50% over that possessed by a non-vanadium containing steel having a similar composition. Moreover, where the vanadium content is increased to about 0.25% the impact strength more than doubles that of a non-vanadium steel This high level of impact strength is maintained where the vanadium content has been increased up to 1.0%. Further increases in the vanadium content do not appear to provide any further increases to the impact strength, nor the tensile properties exhibited by the steel containing vanadium within the claimed range. The optimum results appear to be obtained where the vanadium content is maintained within the range between about 0.25% and about 0.8%.

The balance of the steel of the present invention comprises essentially iron and normal steel mill impurities. In this respect, it is desirable to maintain the impurity content of these compositions at relatively low levels, that is, it is desirable to maintain the sulfur content as low as feasibly possible since it was found that Where the sulfur content is maintained at less than about 0.01%, the steel will have approximately 40% greater impact strength than if the sulfur content is at about 0.02% to about 0.025%. Thus, it is desirable to maintain the sulfur content as low as feasibly possible. In this same vein, it is also desirable to control the unintentional addition of such carbide formers as titanium and zirconium. As little as 0.17% titanium has been found to be extremely deleterious to the steel of the present invention, in comparison with the beneficial effect attributable to vanadium as will be set forth more fully hereinafter. Moreover, it has been found that while tungsten can be tolerated in amounts of about 0.15% and columbium in amounts up to about 0.26%, high tungsten, columbium and chromium have adverse effects on the impact strength.

Reference is directed to Table I which sets forth the chemical composition of the steel of the present invention in terms of a general range, and in terms of an optimum range.

TABLE I Element General Range,

Optimum Range, percent by wt.

percent by wt.

Up to .03. Low as practical.

About 9. About 7. About 5. .25.8.

Balance.

The steel of the present invention may be made in any of the well-known steel mill manners, the details of which are well-known to those skilled in the art and need not be set forth in detail. It is sufiicient to say, that normal melting techniques are employed and the steel is processed from ingot into a variety of shapes and forms, including both bar and flat rolled products. The most significant advantages, however, appear to be maintained where the material is used in heavier gauges, that is, in bar and plate forms.

The steel of this invention, when fabricated into the desired component, is subjected to a heat treatment for developing its optimum mechanical properties. This heat treatment includes annealing at a temperature within a range between about 1450 F. and about 1650 F. for a time period of up to about one hour. Thereafter, the steel is quickly cooled to room temperature, such cooling being effected either by oil or water, or where desired, air cooling may be employed. Since there is an exceedingly low carbon content, the steel in the annealed condition will possess a hardness of less than about R Following the annealing heat treatment, the steel can be fabricated and the fabricated article of manufacture is subjected to a low temperature aging treatment within the range between about 900 F. and 1100 F. for a time period of about four to sixteen hours following which the steel may be air cooled. The low temperature aging treatment is quite beneficial for minimizing distortion of the fabricated article of manufacture during heat treatment, and results in a minimum of scaling t-o the fabricated part. Where desired, a controlled atmosphere may be employed which will minimize any scaling and at the same time prevent any gas phase deposition.

In order to more clearly demonstrate the effects of some of the alloying components, references directed to Table II which list the chemical composition of a series of heats which have been made and tested, and which illustrate various facets of the present invention.

TABLE II Chemical Composition (percent by wt.)

Heat No.

Ni Mo D thereafter these specimens were tested both in the annealed and age hardened condition.

Reference is directed to Table III which sets forth such tensile and impact test results.

TABLE III Element, Yield Tensile Elong., Bed. of A, Hardness, V-Notch, Heat No. Percent Strength, Strength, Percent; Percent Re ft.-lbs.

k.p.s.i. k.p.s.i.

A. Effect of Carbon:

B. Etfect of Sum of Manganese and Silicon:

From the test result recorded in section A of Table III, it can be seen that increasing the carbon content from about 0.04% up to about in a steel containing between .002% and .007% manganese, .016% and 084% silicon, 9.0% and 9.07% nickel, 7.0% and 7.05% cobalt, 4.95% and 5.0% molybdenum, and the balance essentially iron, demonstrates a progressively decreasing level of attainable mechanical properties especially as manifested by the yield strength, tensile strength, hardness and impact strength. While the impact strength of steels containing less than about 0.1% carbon are attractive, nonetheless, substantially greater impact strengths can be achieved where the carbon content is maintained at less than about 0.05% carbon and the steel contains between .25 and 1.0% vanadium. In this series of tests, vanadium was deliberately omitted so as not to mask the overall eifect of carbon. Where the optimum vanadium content would be present, it would be expected that increases in the carbon content in substantially the same increments would result in inferior yield strength and tensile strength, as well as hardness and impact strength.

Referring now to section B of Table III, the effect of the sum of the manganese and silicon is demonstrated at two carbon levels, it being noted that these steels contain vanadium content within the optimum range. The steels set forth therein had a composition which included between 9.0% and 9.05% nickel, 7.0% and 7.01% cobalt, 5.0% and 5.15% molybdenum, 0.27% and 0.28% vanadium and with the carbon and manganese plus silicon contents as noted in Table III. At the 02% carbon level, as demonstrated by heats 872 and 1108, it is seen that increasing the sum of the manganese and silicon contents up to about 0.40% while eifective for increasing the attainable level of tensile properties, seriously adversely affects the impact strength. This same trend is noted when comparing heats 1110 with 1109 wherein substantially the same levels of the sum of manganese and silicon are employed but at the 0.05% carbon level. It is significant to point out that with the increase in the carbon, the attainable level of impact strength is lower than that for the 0.02% carbon level, nonetheless, where the manganese and silicon are maintained within the ranges stated, excellent tensile properties are obtained in combination with outstanding impact strength. Where, however, the manganese and silicon are increased to about .4% or greater, the impact strength is greatly impaired.

A comparison of heats 350, 1492, 872, 873 and 1493 demonstrates the outstanding advantage attributable to the presence of vanadium within the range given for the steel of the present invention. These heats have a composition which includes between 0.01% and 0.03% carbon, 002% and .14% manganese, 014% and 028% silicon, 9.0% and 9.10% nickel, 6.87% and 7.0% cobalt, 4.98% and 5.01% molybdenum and a ranging vanadium content as set forth in Table III. While no outstanding effect is noted on the level of the attainable tensile properties nor on the hardness, nonetheless, the improvement on the impact strength is outstanding. Since these mechanical properties, that is tensile properties and hardness are substantially unaffected with increasing vanadium contents and since the outstanding effect of vanadium is noted in the impact strength, it is believed that vanadium has 0 not entered into the carbide forming relationship in the present steel.

The steel of the present invention requires no special sldlls in the melting or processing thereof, to obtain the semi-finished mill product from which a variety of articles may be manufactured. The simplified heat treatments are effective for producing the required degree of mechanical properties at a minimum of scaling and distortion to the fabricated articles.

I claim:

1. A high strength maraging steel consisting essentially of up to about 0.05% carbon, up to 0.3% manganese, up to about 0.2% silicon, from about 8% to 12% nickel, about 6% to 9% cobalt, about 3.3% to 6.6% molybdenum, from 0.10% to 1.0% vanadium, and the balance essentially iron with incidental impurities.

2. A high strength maraging steel consisting essentially of up to about 0.05% carbon, not more than 0.3% manganese, not more than about 0.2% silicon, the sum of the silicon plus manganese not exceeding about 0.4%, about 9% nickel, about 7% cobalt, about 5% molybdenum, from 0.10% to 1% vanadium, and the balance essentially iron with incidental impurities.

3. A high strength maraging steel consisting essentially of up to about 0.05% carbon, not more than about 0.3% manganese, not more than about 0.2% silicon, the sum of the silicon plus manganese not exceeding about 0.4%, about 9% nickel, about 7% cobalt, about 5% molybdenum, from 0.25% to 0.8% vanadium, and the balance essentially iron with incidental impurities.

4. An article of manufacture characterized by possessing an aged martensitic microstructure, a tensile strength of about 200 k.p.s.i., good ductility, excellent impact strength and a composition consisting essentially of up to about 0.05% carbon, less than about 0.2% manganese, less than about 0.1% silicon, about 9% nickel, about 7% cobalt, about 5% molybdenum, from 0.10% to 1.0% vanadium, and the balance essentially iron with incidental impurities.

5. An article of manufacture characterized by possessing an aged martensitic microstructure, a tensile strength of about 200 k.p.s.i., good ductility, excellent impact strength and a composition consisting essentially of up to about 0.05% carbon, less than about 0.2% manganese, less than about 0.1% silicon, about 8% to 12% nickel, about 6% to 9% cobalt, about 3.3% to 6.6% molybdenum, from 0.10% to 1.0% vanadium and the balance essentially iron with incidental impurities.

References Cited by the Examiner UNITED STATES PATENTS 2,865,740 12/1958 Heger et al -123 3,093,519 6/1963 Decker et al. 75-123 X 3,132,937 5/1964 Sadowski et al. 75-124 3,132,938 5/1964 Decker et al l4-8l42 X 3,166,406 1/1965 Floreen et al. 148142 X 3,243,285 3/1966 Fragetta et al 75-423 HYLAND BIZOT, Primary Examiner. PAUL WEINSTEIN, Assistant Examiner. 

4. AN ARTICLE OF MANUFACTURE CHARACTERIZED BY POSSESSING AN AGED MARTENSITIC MICROSTRUCTURE, A TENSILE STRENGTH OF ABOUT 200 K.P.S.I., GOOD DUCTILITY, EXCELLENT IMPACT STRENGTH AND A COMPOSITION CONSISTING ESSENTIALLY OF UP TO ABOUT 0.05% CARBON, LESS THAN ABOUT 0.2% MANGANESE, LESS THAN ABOUT 0.1% SILICON, ABOUT 9% NICKEL, ABOUT 7% COBALT, ABOUT 5% MOLYBDENUM, FROM 0.10% TO 1.0% VANADIUM, AND THE BALANCE ESSENTIALLY IRON WITH INCIDENTAL IMPURITIES. 